Shaped charge and explosively formed penetrator (hereinafter referred to as SC/EFP) devices are used to develop holes in, and/or penetrate hard structures. SC/EFP devices incorporate a liner fabricated from pure metals, alloys, and/or ceramics which typically include elements such as chromium, copper, molybdenum, tantalum, tungsten, rhenium, osmium, niobium, platinum, iridium, hafnium, and uranium. It is the explosive formation of high velocity jets (SC's) and high velocity slugs (EFP's) of these metal and ceramic liners that form the penetrators capable of breaking through rock and other hard materials. Examples of such devices are disclosed in U.S. Pat. No. 4,498,367 by Skolnick et al.; U.S. Pat. No. 4,551,287 by Bethmann; U.S. Pat. No. 4,841,864 by Grace; and U.S. Pat. No. 4,958,569 by Mandigo. Each of these U.S. patents is incorporated herein by reference in their entirety.
The stability of the high velocity metal jet/slug determines the efficiency with which the target is penetrated. A highly stable, elongated jet exhibits superior penetration performance as compared to an unstable, short, segmented jet. The formation of high velocity jets via explosive forming is dependent upon a variety of material properties inherent in the base material of the SC/EFP liner. Favorable properties include, but are not limited to, high melting temperature, high density, high bulk speed of sound, fine grain size, proper grain orientation, good elongation, minimal fabrication imperfections, low impurity content, high dynamic strength and high dynamic toughness. Some salient properties for a variety of potential SC/EFP liner materials are illustrated in Table I.
TABLE I ______________________________________ Potential High Melting Point SC/EFP Liner Materials Material Melting Point (.degree. C.) Density (g/cm.sup.3) Crystal Structure ______________________________________ W 3407 19.3 BCC Os 3027 22.4-22.7 Hexagonal Ta 3014 16.6 BCC Mo 2618 10.2 BCC Nb 2467 8.55 BCC Ir 2443 22.5 FCC Ru 2250 12.2 Hexagonal Hf 2227 13.1 Hexagonal Re 1964 21.0 Hexagonal V 1902 5.80 BCC Cr 1857 7.19 BCC Pt 1772 21.4 FCC Th 1755 11.7 FCC Ti 1669 4.50 Hexagonal Fe 1536 7.86 BCC U 1132 18.9 Orthorhombic ______________________________________
The probability of high dynamic ductility is greater in BCC and FCC metals due to the presence of more slip systems in these lattices than in hexagonal lattices.
Current SC/EFP liners exhibit limitations due to material property constraints. Current manufacturing techniques for SC/EFP liners include the following: 1) casting processes; 2) forming processes, including powder metallurgy techniques, hot working techniques and cold working techniques; 3) machining processes; and 4) other techniques such as grinding and metallizing. In particular, current technologies for the formation of liners are believed to limit the minimum grain size in the liner to between about 5 and 100 micrometers, depending on the specific material. Finer grained SC/EFP liners would exhibit enhanced performance resulting from the formation of a more stable jet. However, materials with extremely fine grain size are generally not available for this purpose.
SC/EFP liners with a submicron grain size have been fabricated by a chemical vapor deposition (CVD) process, specifically by forming liners of tungsten and rhenium using tungsten hexafluoride and rhenium hexafluoride. Although these liners have a fine grain size, they possess chemical impurities that produce deleterious effects on the jet formation from the liner. In addition, the CVD process is, in general, quite slow and expensive.
In addition to the limitations discussed above, current manufacturing technologies frequently require expensive machining steps to produce the high precision metal liners for SC/EFP devices. In particular, many processes for the production of liners from higher density materials currently require the removal of large quantities of metal after the liner is first produced. It is estimated that about 80% of the cost associated with the formation of tungsten shaped charge liners is associated with the machining process.
Still another material limitation pertains to variations in the microstructure of forgings used to fabricate SC/EFP liners. This lack of a uniform starting material results in inconsistencies in the performance of the liners. EFP liners made by slicing disks of metal from a forging will have microstructural differences based upon the location of the slice in the forging. These positional differences can cause major performance differences in the functioning of the device. The fabrication of liners via the forging approach is also limited due to difficulties in obtaining forgings of the appropriate size for metals such as molybdenum and tungsten.